Interstellar Dust and Gas

In 1783 William Herschel began a survey of theheavens using an 18 ¾ inch reflector of his ownconstruction. His goal was to discover new starclusters, nebulae, and double stars. He made countsof the numbers of stars seen in his 15 arcmin fieldof view and eventually produced this model of ourGalaxy:

To his surprise, when surveying the sky between declination -22 and -24 deg near Scorpius, he founda region devoid of stars. He said to his sister, who was his observing assistant, “Hier ist wahrhaftig einLoch im Himmel.” (Here is indeed a hole in the heavens.) The object he was referring to is knownas (rho) Ophiuchi.

Other discoveries that provided evidence that therewas material distributed between the stars:

1847 – W. Struve publishes Etudes d'AstronomieStellaire. From the number of stars per magnitudebin, he concludes that there must be somethingdistributed through space that is dimming starlight

1904 – J. F. Hartmann studied the right hand starin the belt of Orion (Ori). It is a spectroscopicbinary composed of an O 9.5 bright giant, a muchfainter star, plus a visual companion 52 arcsec away.The O-type star is too hot to have calcium lines inits spectrum, but Ca lines are seen nevertheless.

While the spectral lines of the O-type star shift backand forth as the star and its faint companion orbittheir center of mass, the Ca lines in the spectrum donot move. The Ca lines are due to interstellar gasalong the line of sight.

1930 – R. J. Trumpler's studies of open star clustersinvolved determining distances from the angularsizes of the clusters and also from the brightness ofthe stars. As we proceed to further clusters, theirphotometric distances deviate more and more fromthe geometric distances. Either the light was fallingoff faster than the square of the distance, or somethingwas dimming the light of the stars.

We are now certain that the plane of our Galaxy contains gas and dust. The gas can be detected ata variety of wavelengths.

The dust is detected from its effect on

star counts

dimming the light of stars

reddening the light of stars

It is possible that a star in our Galactic plane isdimmed 3 magnitudes in the V-band (550nm),and 4 magnitudes in the B-band (440 nm). Sayit is an A-type star like Vega. Instead of havinga B-V color of 0.00, it will be 1 magnitude redder and will have the color of a K-type starlike Arcturus.

Such a star would be dimmed by only 0.3 magin the K-band (2.2 microns in the near-IR).At far infrared wavelengths the interstellardust would have only a minor effect on thebrightness.

Because interstellar extinction maximizes at wavelengths of 0.2 microns and less, the size ofthe interstellar grains must be roughly that size.

Interstellar extinction by dust is 10 times lessserious at a wavelength of 2.2 microns in the near-infrared compared to extinction in the V-band at 0.55 microns (550 nm).

If we know the intrinsic colors of certain kinds ofstars (by spectral type, for example), we can usethe observed colors to determine how much theyhave been reddened by interstellar dust.

SN 1999clis dimmed2 magnitudesat visual wavelengthsby dust inits hostgalaxy.

V-K colorsof someType Iasupernovae.98bu and99cl havebeen reddenedby dust intheir hostgalaxies

much redder

redder

As we will find out shortly, stars are formedfrom the gravitational collapse of dust and gasclouds. The dust traps the optical and ultravioletlight from any hot stars that form. If we want tosee what is happening in a star forming region,we must take images at infrared and radio wavelengths,because a much higher percentage of longer wavelength light is able to escape from the star forming region.

Infrared telescopes such as the United Kingdom InfraredTelescope and the Spitzer Space Telescope are key facilities for studying star formation.

Radio wave photons at = 21 cm are produced whenthe electrons of atomic hydrogen change their spin.

Neutral hydrogen inour Galaxy is found in the planeof the Galaxy andis associated withthe spiral arms.

Three Kinds of Nebulae

emission nebulae – ionized by stars with temperatures greater than 25,000 K

reflection nebulae – starlight scattered of a dusty nebula

dark nebulae – dense clouds of dust that obscure the light of more distant stars

The Eagle Nebula,as imaged withthe Hubble SpaceTelescope.

A list of more than 100 interstellar molecules can befound at:

www.cv.nrao.edu/~awootten/allmols.html

Two examples:

C7H

HC11

N

One molecule that mightbe out there is C

60. It is

a member of a family ofmolecules called fullerenesafter the American architectR. Buckminster Fuller(1895-1983), who advocatedthat we live in houses in theshape of geodesic domes.